1. Field of the Art
The present invention relates to a method for aligning two pipeline ends, preferably two pipeline ends located on the bottom of the sea, by using a frame open towards the bottom, which frame is brought in position above the pipeline ends and which is equipped with means for displacing and aligning the pipe ends.
The invention also relates to an apparatus for carrying out the method.
2. Statement of Prior Art
The alignment of two pipeline ends is an operation which often occurs in connection with laying and/or repair of pipelines in deep water. However, it is to be understood that the method and apparatus according to the invention not necessarily is associated with subsurface pipelines, but also with pipelines following a line ashore. However, the method and the apparatus of the invention will be described with reference to sub-surface pipelines, the invention primarily being contemplated for use in marine environment.
Pipelines which are laid out in deep water will generally be laid out in long continuous lengths. However, during the laying operation bad weather may occur, so that the laying must be interrupted and when commencing the laying the new pipeline must be jointed to the already installed pipe. From the laying vessel it is, however, difficult to lay out the new pipe length exactly in line with and close to the already installed pipe length, and in most cases of resumed pipe laying there will occur an angular deviation between and an overlapping of the two ajoining pipeline ends.
When the pipe ends are to be jointed, e.g. by welding, the pipe ends are cut so that the distance therebetween adopts a suitable length, e.g., 1/2-1 meter, whereafter the pipe ends are aligned relative to each other and welded together by an intermediate pipe piece.
When a damaged pipeline is to be repaired it is usual to cut and remove the damaged pipe section, which thereafter is replaced by a new pipe piece which is welded to the respective pipe ends.
In connection with both the mentioned conditions it is known to use an alignment rig or frame, which usually comprises a welded, trussed structure having clamps or gripping means gripping around the pipe ends and moving these by hydraulic devices. The power to the driving means is supplied through a cable from a surface vessel which also carries the alignment frame prior to the lowering thereof to the jointing area of the pipe ends.
Most of the pipe alignment rigs have very large dimensions and are generally equipped with means serving to affix the rig to the bottom of the sea.
Rigs for use in the alignment of pipe ends at the bottom of the sea are disclosed, e.g. in Norwegian Pat. NP No. 134 315, U.S. Pat. No. 3,508,410 and West German Pat. No. 1 934 884. Thus from Norwegian Pat. No. 134 315 there is known a device for use in connection with the jointing of pipes at the bottom of the ocean, comprising a frame open towards the bottom, which frame is lowered from a carrier vessel to bridge the pipeline ends, and which is equipped with displacable gripping means which can grip around the pipe ends and by hydraulic jacks displace the ends relative to each other to a suitable welding position. However, the hydraulic jacks provided on the frame according to Norwegian Pat. No. 134 315 have a very limited stroke compared to the width of the frame or the rig, the clamps gripping around the pipe ends being arranged with pairs of opposite power cylinders which combined occupy more than one third of the width of the frame. In connection with pipe ends which are spaced far apart and which therefore require a rig having hydraulic power cylinders with a long stroke, a rig constructed according to the teaching of Norwegian Pat. No. 134 315 must be made disproportionately large, the weight of the rig increasing considerably. This will further entail that the crane equipment on the surface vessel must be strengthen even more to cope with the raising and lowering of the large weights represented by such a known pipe positioning rig.
In Norwegian Pat. No. 134 315 even the use of a frame which favourably has a very substantial weight, is suggested, so as to provide a stable foundation. To achieve additional stability of the frame the use of piles affixing the frame to the bottom of the ocean have been suggested according to said patent specification. However, such an affixing of the frame will render a setting-up of the pipes which to a substantial degree depends on the fixed working position adopted by the frame relative to the pipe ends. Such a setting-up may give unwanted, large deformations in the pipe ends, which is unfavourable to a later welding operation.